Rational design and assembly of M₂M′₃L₆ supramolecular clusters with C_3h symmetry by exploiting incommensurate symmetry numbers

A rational approach to heterometallic cluster formation is described that uses incommensurate symmetry requirements at two different metals to control the stoichiometry of the assembly. Critical to this strategy is the proper design and synthesis of hybrid ligands with coordination sites selective toward each metal. The phosphino-catechol ligand 4-(diphenylphosphino)benzene-1,2-diol (H₂L) possesses both hard catecholate and soft phosphine donor sites and serves such a role, using soft (C₂-symmetric) and hard (C₃-symmetric) metal centers. The ML₃ catecholate complexes (M = Fe^III, Ga^III, Ti^IV, Sn^IV) have been prepared and characterized as C₃-symmetry precursors for the stepwise assembly (aufbau) of heterometallic clusters. While the single-crystal X-ray structure of the Cs₂ [TiL₃] salt shows a C₁ mer-configuration in the solid-state, room-temperature solution NMR data of this and related complexes are consistent with either exclusive formation of the C₃-fac-isomer with all PPh₂ donor sites syn to each other or facile fac/mer isomerization. Coordination of these [ML₃]^2- (M = Ti^IV, Sn^IV) metallaligands via their soft P donor sites to C₂-symmetric PdBr₂ units gives exclusively pentametallic [M₂Pd₃Br₆L₆]^4- (M = Ti, Sn) clusters. These clusters have been fully characterized by spectral and X-ray structural data as C_3h mesocates with Cs⁺ or protonated 1,4-diazabicyclo-[2.2.2]octane (DABCO·H⁺) cations incorporated into deep molecular clefts. Exclusive formation of this type of supramolecular species is sensitive to the nature of the counterions. Alkali cations such as K⁺, Rb⁺, and Cs⁺ give high-yield formation of the respective clusters while NEt₃H⁺ and NMe₄⁺ yield none of the desired products. Extension of the aufbau assembly to produce related [M₂Pd₃Cl₆L₆]^4-, [M₂Pd₃I₆L₆]^4-, and [M₂Cr₃(CO)₁₂L₆]^4- (M = Ti, Sn) clusters has also been realized. In addition to this aufbau approach, self-assembly of several of these [M₂Pd₃Br₆L₆]^4- clusters from all eleven components (two M^IV, three PdBr₂, six H₂L) was also accomplished under appropriate reaction conditions.